Chapter 21

CHAPTER 21
SOIL IMPROVEMENT
21.1 INTRODUCTION
General practice is to use shallow foundations for the foundations of buildings and other such
structures, if the soil close to the ground surface possesses sufficient bearing capacity. However,
where the top soil is either loose or soft, the load from the superstructure has to be transferred to
deeper firm strata. In such cases, pile or pier foundations are the obvious choice.
There is also a third method which may in some cases prove more economical than deep
foundations or where the alternate method may become inevitable due to certain site and other
environmental conditions. This third method comes under the heading foundation soil
improvement. In the case of earth dams, there is no other alternative than compacting the remolded
soil in layers to the required density and moisture content. The soil for the dam will be excavated at
the adjoining areas and transported to the site. There are many methods by which the soil at the site
can be improved. Soil improvement is frequently termed soil stabilization, which in its broadest
sense is alteration of any property of a soil to improve its engineering performance. Soil
improvement
1. Increases shear strength
2. Reduces permeability, and
3. Reduces compressibility
The methods of soil improvement considered in this chapter are
1. Mechanical compaction
2. Dynamic compaction
3. Vibroflotation
4. Preloading
5. Sand and stone columns
951

952 Chapter 21
6. Use of admixtures
7. Injection of suitable grouts
8. Use of geotextiles
21.2 MECHANICAL COMPACTION
Mechanical compaction is the least expensive of the methods and is applicable in both
cohesionless and cohesive soils. The procedure is to remove first the weak soil up to the depth
required, and refill or replace the same in layers with compaction. If the soil excavated is
cohesionless or a sand-silt clay mixture, the same can be replaced suitably in layers and
compacted. If the soil excavated is a fine sand, silt or soft clay, it is not advisable to refill the same
as these materials, even under compaction, may not give sufficient bearing capacity for the
foundations. Sometimes it might be necessary to transport good soil to the site from a long
distance. The cost of such a project has to be studied carefully before undertaking the same.
The compaction equipment to be used on a project depends upon the size of the project and
the availability of the compacting equipment. In projects where excavation and replacement are
confined to a narrow site, only tampers or surface vibrators may be used. On the other hand, if the
whole area of the project is to be excavated and replaced in layers with compaction, suitable roller
types of heavy equipment can be used. Cohesionless soils can be compacted by using vibratory
rollers and cohesive soils by sheepsfoot rollers.
The control of field compaction is very important in order to obtain the desired soil
properties. Compaction of a soil is measured in terms of the dry unit weight of the soil. The dry
unit weight, yd, may be expressed as
1+ w
where,
yt = total unit weight
w = moisture content
Factors Affecting Compaction
The factors affecting compaction are
1. The moisture content
2. The compactive effort
The compactive effort is defined as the amount of energy imparted to the soil. With a soil of
given moisture content, increasing the amount of compaction results in closer packing of soil
particles and increased dry unit weight. For a particular compactive effort, there is only one
moisture content which gives the maximum dry unit weight. The moisture content that gives the
maximum dry unit weight is called the optimum moisture content. If the compactive effort is
increased, the maximum dry unit weight also increases, but the optimum moisture content
decreases. If all the desired qualities of the material are to be achieved in the field, suitable
procedures should be adopted to compact the earthfill. The compactive effort to the soil is imparted
by mechanical rollers or any other compacting device. Whether the soil in the field has attained the
required maximum dry unit weight can be determined by carrying out appropriate laboratory tests
on the soil. The following tests are normally carried out in a laboratory.
1. Standard Proctor test (ASTM Designation D-698), and
2. Modified Proctor test (ASTM Designation D-1557)

Soil Improvement 953
21.3 LABORATORY TESTS ON COMPACTION
Standard Proctor Compaction Test
Proctor (1933) developed this test in connection with the construction of earth fill dams in
California. The standard size of the apparatus used for the test is given in Fig 21.1. Table 21.1
gives the standard specifications for conducting the test (ASTM designation D-698). Three
alternative procedures are provided based the soil material used for the test.
Test Procedure
A soil at a selected water content is placed in layers into a mold of given dimensions (Table 21.1
and Fig. 21.1), with each layer compacted by 25 or 56 blows of a 5.5 Ib (2.5 kg) hammer dropped
from a height of 12 in (305 mm), subjecting the soil to a total compactive effort of about 12,375
fl-lb/ft3
(600 kNm/m3
). The resulting dry unit weight is determined. The procedure is repeated for
a sufficient number of water contents to establish a relationship between the dry unit weight and
the water content of the soil. This data, when, plotted, represents a curvilinear relationship known
as the compaction curve or moisture-density curve. The values of water content and standard
maximum dry unit weight are determined from the compaction curve as shown in Fig. 21.2.
Item
Table 21.1 Specification for standard Proctor compaction test
Procedure
A B
1. Diameter of mold
2. Height of mold
3. Volume of mold
4. Weight of hammer
5. Height of drop
6. No. of layers
7. Blows per layer
8. Energy of
compaction
9. Soil material
4 in. (101.6 mm)
4.584 in. (116.43 mm)
0.0333 ft3
(944 cm3
)
5.5 Ib (2.5 kg)
12.0 in. (304.8 mm)
3
25
12,375 ft-lb/ft3
(600 kN-m/m3
)
Passing No. 4 sieve
(4.75 mm). May be used
if 20% or less retained
on No. 4 siere
4 in. (101.6 mm)
4.584 in. (116.43 mm)
0.0333 ft3
(944 cm3
)
5.5 Ib (2.5 kg)
12.0 in. (304.8 mm)
3
25
12,375 ft-lb/ft3
(600 kN-m/m3
)
Passing No 4 sieve (4.75 mm).
Shall be used if 20% or more
retained on No. 4 sieve and
20% or less retained on
3/8 in (9.5 mm) sieve
6 in. (152.4 mm)
4.584 in. (116.43 mm)
0.075 ft3
(2124 cm3
)
5.5 Ib (2.5 kg)
12.0 in. (304.8 mm)
3
56
12,375 ft-lb/ft3
(600 kN-m/m3
)
Passing No. 4 sieve
(4.75 mm). Shall be
used if 20% or more
retained on 3/8 in.
(9.5 mm ) sieve and
less than 30% retained
on 3/4 in. (19 mm) sieve

954 Chapter 21
6cm
11.7cm
101.6mm
or 152.4 mm
Collar
Metal mold
Hammer
Detachable base plate —J
h = 30 or 45 cm W= 2.5 or 4.5 kg
(a)
5 cm
Figure 21.1 Proctor compaction apparatus: (a) diagrammatic sketch, and (b)
photograph of mold, and (c) automatic soil compactor (Courtesy: Soiltest)
Modified Proctor Compaction Test (ASTM Designation: D1557)
This test method covers laboratory compaction procedures used to determine the relationship
between water content and dry unit weight of soils (compaction curve) compacted in a 4 in. or 6
in. diameter mold with a 10 Ib (5 kg) hammer dropped from a height of 18 in. (457 mm)
producing a compactive effort of 56,250 ft-lb/ft3
(2,700 kN-m/m3
). As in the case of the standard
test, the code provides three alternative procedures based on the soil material tested. The details
of the procedures are given in Table 21.2.

Table 21.2 Specification for modified Proctor compaction test
Item Procedure
B
1.
2.
3.
4.
5.
6.
7.
8.
Mold diameter
Volume of mold
Weight of hammer
Height of drop
No. of layers
Blows / layer
Energy of compaction
Soil material
4 in. (101.6mm)
0.0333 ft3
(944cm3
)
10 Ib (4.54 kg)
18 in. (457.2mm)
5
25
56,250 ft lb/ft3
(2700 kN-m/m3
)
May be used if 20% or
less retained on No. 4
sieve.
4 in. (101.6mm)
0.0333 ft3
(944 cm3
)
10 Ib (4.54 kg)
18 in. (457.2mm)
5
25
56,250 ft lb/ft3
(2700 kN-m/m3
)
Shall be used if 20%
or more retained on
No. 4 sieve and
20% or less retained
on the 1/8 in. sieve
6 in. (101.6mm)
0.075 ft3
(2124 cm3
)
101b(4.54kg)
18 in. (457.2mm)
5
56
56,250 ft lb/ft3
(2700 kN-m/m3
)
Shall be used if more-
than 20% retained on
3/8 in. sieve and less-
than 30% retained on
the 3/4 in. sieve
(19 mm)
Test Procedure
A soil at a selected water content is placed in five layers into a mold of given dimensions, with
each layer compacted by 25 or 56 blows of a 10 Ib (4.54 kg) hammer dropped from a height of 18
in. (457 mm) subjecting the soil to a total compactive effort of about 56,250 ft-lb/ft3
(2700 kN-m/m3
). The resulting dry unit weight is determined. The procedure is repeated for a
sufficient number of water contents to establish a relationship between the dry unit weight and the
water content for the soil. This data, when plotted, represents a curvilinear relationship known as
the compaction curve or moisture-dry unit weight curve. The value of the optimum water content
and maximum dry unit weight are determined from the compaction curve as shown in Fig. 21.2.
Determination of Zero Air Voids Line
Referring to Fig. 21.3, we have
O W
Degree of saturation, y
Water content, w —
W
Dry weight of solids, Ws = VSGS yw = Gs yw since Vs = 1
W wG Y
y
Therefore
yw rw
wGs
V (21.2)

956 Chapter 21
2.2
2.1
2.0
1.7
1.6
Max. dry unit wei|
Max. dry
• unit weight
I _l
90% saturation curve
I !
^ 95%saturationcurve
Opt. moisturecontent
100% saturationline
(zero air voids)
8 12 16 20 24
Moisture content, w, percent
28 32
Figure 21.2 Moisture-dry unit weight relationship
or V. =
wG
Dry unit weight
W G y
s ' w
(21.3)
1+
In Eq. (21.3), since G and y , remain constant for a particular soil, the dry unit weight is a
function of water content for any assumed degree of saturation. If S = 1, the soil is fully saturated
(zero air voids). A curve giving the relationship between yd and w may be drawn by making use of
Eq. (20.3) for 5=1. Curves may be drawn for different degrees of saturation such as 95, 90, 80 etc
Air
Water
Solids
Figure 21.3 Block diagram for determining zero air voids line

percents. Fig. 21.2 gives typical curves for different degrees of saturation along with moisture-dry
unit weight curves obtained by different compactive efforts.
Example 21.1
A proctor compaction test was conducted on a soil sample, and the following observations were
made:
Water content, percent 7.7 11.5 14.6 17.5 19.7 21.2
Mass of wet soil, g 1739 1919 2081 2033 1986 1948
If the volume of the mold used was 950 cm3
and the specific gravity of soils grains was 2.65,
make necessary calculations and draw, (i) compaction curve and (ii) 80% and 100% saturation
lines.
Solution
From the known mass of the wet soil sample and volume of the mold, wet density or wet unit
weight is obtained by the equations,
/ . M Mass ofwetsample ingm , ,
/0,(g/cm3
) = — = — f 2—oryt = (kN/m3
) = 9.81xp, (g/cm3
)
V 950 cm-
Then from the wet density and corresponding moisture content, the dry density or dry unit
weight is obtained from,
/>y=— or
rrf= —
d
l + w d
l +w
Thus for each observation, the wet density and then the dry density are calculated and
tabulated as follows:
Water content, percent 7.7 11.5 14.6 17.5 19.7 21.2
Mass of wet sample, g 1739 1919 2081 2033 1986 1948
Wet density, g/cm3
1.83 2.02 2.19 2.14 2.09 2.05
Dry density, g/cm3
1.70 1.81 1.91 1.82 1.75 1.69
Dry unit weight kN/m3
16.7 17.8 18.7 17.9 17.2 16.6
Hence the compaction curve, which is a plot between the dry unit weight and moisture
content can be plotted as shown in the Fig. Ex. 21.1. The curve gives,
Maximum dry unit weight, MDD = 18.7 kN/m3
Optimum moisture content, OMC = 14.7 percent
For drawing saturation lines, make use of Eq. (21.3), viz.,
wG
1 + s
-
where, G^ = 2.65, given, S = degree of saturation 80% and 100%, w = water content, may be
assumed as 8%, 12%, 16%, 20% and 24%.

958 Chapter 21
22
21
20
19
18
17
16
15
5= 100%
10 /X
15
0MC = 14.7%
Moisture content, %
Fig. Ex. 21.1
20
Hence for each value of saturation and water content, find }g, and tabulate:
Water content, percentage 8 12 16 20 24
7^ kN/m3
for 5 = 100%
yd kN/m3
for 5 = 80%
21.45 19.73 18.26 17.0 15.69
20.55 18.61 17.00 15.64 14.49
With these calculations, saturation lines for 100% and 80% are plotted, as shown in the
Fig. Ex. 21.1.
Also the saturation, corresponding to MOD = 18.7 kN/m3
and OMC = 14.7% can be
calculated as,
Gsyw _ 2.65x9.81
18-7 = =
H'GV
S
1 +
0.147x2.65
S
which gives S = 99.7%
Example 21.2
A small cylinder having volume of 600 cm3
is pressed into a recently compacted fill of
embankment filling the cylinder.The mass of the soil in the cylinder is 1100 g. The dry mass of the
soil is 910 g. Determine the void ratio and the saturation of the soil. Take the specific gravity of the
soil grains as 2.7.
Solution
Wet density of soil
Water content, w =
1100
600
1100-910 190
910
1.83 g/cm3
or y, - 17.99 kN/m3
= 0.209 = 20.9%

Y 1799
Dry unit weight, v , = -LL
- = -:
- =14.88 kN/m3
5 d
l + w 1 +0.209
From, y. = - - we have e =
yd
Substituting and simplifying
e= _! =0.78
14.88
r wG
s 0.209x2.7x100 ^^
From, Se= wG<, or 5 = --= - =72.35%
e 0.78
21.4 EFFECT OF COMPACTION ON ENGINEERING BEHAVIOR
Effect of Moisture Content on Dry Density
The moisture content affects the behavior of the soil. When the moisture content is low, the soil is
stiff and difficult to compress. Thus, low unit weight and high air contents are obtained (Fig. 21.2).
As the moisture content increases, the water acts as a lubricant, causing the soil to soften and
become more workable. This results in a denser mass, higher unit weights and lower air contents
under compaction. The water and air combination tend to keep the particles apart with further
compaction, and prevent any appreciable decrease in the air content of the total voids, however,
continue to increase with moisture content and hence the dry unit weight of the soil falls.
To the right of the peak of the dry unit weight-moisture content curve (Fig. 21.2), lies the
saturation line. The theoretical curve relating dry density with moisture content with no air voids is
approached but never reached since it is not possible to expel by compaction all the air entrapped in
the voids of the soil.
Effect of Compactive Effort on Dry Unit Weight
For all types of soil with all methods of compaction, increasing the amounts of compaction, that is,
the energy applied per unit weight of soil, results in an increase in the maximum dry unit weight
and a corresponding decrease in the optimum moisture content as can be seen in Fig. 21.4.
Shear Strength of Compacted Soil
The shear strength of a soil increases with the amount of compaction applied. The more the soil is
compacted, the greater is the value of cohesion and the angle of shearing resistance. Comparing the
shearing strength with the moisture content for a given degree of compaction, it is found that the
greatest shear strength is attained at a moisture content lower than the optimum moisture content
for maximum dry unit weight. Fig. 21.5 shows the relationship between shear strength and
moisture-dry unit weight curves for a sandy clay soil. It might be inferred from this that it would be
an advantage to carry out compaction at the lower value of the moisture content. Experiments,
however, have indicated that soils compacted in this way tend to take up moisture and become
saturated with a consequent loss of strength.
Effect of Compaction on Structure
Fig. 21.6 illustrates the effects of compaction on clay structure (Lambe, 1958a). Structure (or
fabric) is the term used to describe the arrangement of soil particles and the electric forces between
adjacent particles.

960 Chapter 21
120
115
110
105
~ 100
95
90
85
1 '
2 3
3 c
4 '
/
*
A
Q
rj
X
x^
rr
X
/
<x
'v
°v
^
^
-i"
^
^«——
~
^
N
N
N
"
^
Blows per Hammer Hammer
No. Layers Layer weight(lb) drop (in.)
1 5 5 5 1 0 18 (mod. AASHTO)
2 5 2 6 1 0 1 8
3 5 12 10 18 (std.AASHTO)
4 3 2 5 5'/2 1 2
Note: 6 in. diameter mold used for all tests
Figure 21.4 Dynamic compaction
curves for a silty clay (from Turnbull,
1950)
10 15 20
Water content (%)
1.9
1.7
1.5
Unconflned compressive strength
Moisture-unit weight curve
12 14 16 18
Moisture content percent
2.5
2.0
1.5 >
1.0
20
0.5
Figure 21.5 Relationship
between compaction and
shear strength curves
T3
T3
High compactive
effort
Low
compactive
effort
Molding water content
Figure 21.6 Effects of
compaction on structure
(from Lambe, 1 958a)

The effects of compaction conditions on soil structure, and thus on the engineering
behavior of the soil, vary considerably with soil type and the actual conditions under which the
behavior is determined.
At low water content, WA in Fig. 21.6, the repulsive forces between particles are smaller
than the attractive forces, and as such the particles flocculate in a disorderly array. As the water
content increases beyond WA, the repulsion between particles increases, permitting the particles to
disperse, making particles arrange themselves in an orderly way. Beyond WB the degree of particle
parallelism increases, but the density decreases. Increasing the compactive effort at any given water
content increases the orientation of particles and therefore gives a higher density as indicated in
Fig. 21.6.
Effect of Compaction on Permeability
Fig. 21.7 depicts the effect of compaction on the permeability of a soil. The figure shows the typical
marked decrease in permeability that accompanies an increase in molding water content on the dry
side of the optimum water content. A minimum permeability occurs at water contents slightly
above optimum moisture content (Lambe, 1958a), after which a slight increase in permeability
occurs. Increasing the compactive effort decreases the permeability of the soil.
10-
§10-
io-9
130
126
60
•53 122
118
Q
*-
Shows change in moisture x
(,o
(expansion prevented)
11 13 15
Water content, (%)
17 19
Figure 21.7 Compaction-permeability tests on Siburua clay (from Lambe, 1962)

962 Chapter 21
Dry compacted or
undisturbed sample
Wet compacted or
remolded sample
Stress, natural scale
(a) Low-stress consolidation
Dry compacted or
undisturbed sample
Wet compacted or
remolded sample
Rebound for both samples
Stress, log scale
(b) High-stressconsolidation
Figure 21.8 Effect of one-dimensional compression on structure (Lambe, 1958b)
Effect of Compaction on Compressibility
Figure 21.8 illustrates the difference in compaction characteristics between two saturated clay
samples at the same density, one compacted on the dry side of optimum and one compacted on the
wet side (Lambe, 1958b). At low stresses the sample compacted on the wet side is more
compressible than the one compacted on the dry side. However, at high applied stresses the sample
compacted on the dryside is more compressible than the sample compacted on the wet side.
21.5 FIELD COMPACTION AND CONTROL
The necessary compaction of subgrades of roads, earth fills, and embankments may be obtained by
mechanical means. The equipment that are normally used for compaction consists of
1. Smooth wheel rollers
2. Rubber tired rollers
3. Sheepsfoot rollers
4. Vibratory rollers
Laboratory tests on the soil to be used for construction in the field indicate the maximum dry
density that can be reached and the corresponding optimum moisture content under specified
methods of compaction. The field compaction method should be so adjusted as to translate

Figure 21.9 Smooth wheel roller (Courtesy: Caterpillar, USA)
laboratory condition into practice as far as possible. The two important factors that are necessary
to achieve the objectives in the field are
1. The adjustment of the natural moisture content in the soil to the value at which the field
compaction is most effective.
2. The provision of compacting equipment suitable for the work at the site.
The equipment used for compaction are briefly described below:
Smooth Wheel Roller
There are two types of smooth wheel rollers. One type has two large wheels, one in the rear and a
similar single drum in the front. This type is generally used for compacting base courses. The
equipment weighs from 50 to 125 kN (Fig.21.9). The other type is the tandem roller normally used
for compacting paving mixtures. This roller has large single drums in the front and rear and the
weights of the rollers range from 10 to 200 kN.

964 Chapter 21
Figure 21.10 Sheepsfoot roller (Courtesy: Vibromax America Inc.)
Rubber Tired Roller
The maximum weight of this roller may reach 2000 kN. The smaller rollers usually have 9 to 11
tires on two axles with the tires spaced so that a complete coverage is obtained with each pass. The
tire loads of the smaller roller are in the range of 7.5 kN and the tire pressures in the order of
200 kN/m2
. The larger rollers have tire loads ranging from 100 to 500 kN per tire, and tire pressures
range from 400 to 1000 kN/m2
.
Sheepsfoot Roller
Sheepsfoot rollers are available in drum widths ranging from 120 to 180 cm and in drum diameters
ranging from 90 to 180 cm. Projections like a sheepsfoot are fixed on the drums. The lengths of
these projections range from 17.5 cm to 23 cm. The contact area of the tamping foot ranges from 35
to 56 sq. cm. The loaded weight per drum ranges from about 30 kN for the smaller sizes to 130 kN
for the larger sizes (Fig. 21.10).
Vibratory Roller
The weights of vibratory rollers range from 120 to 300 kN. In some units vibration is produced by
weights placed eccentrically on a rotating shaft in such a manner that the forces produced by the
rotating weights are essentially in a vertical direction. Vibratory rollers are effective for compacting
granular soils (Fig. 21.11).

Figure 21.11 Vibratory drum on smooth wheel roller (Courtesy: Caterpillar, USA)
Selection of Equipment for Compaction in the Field
The choice of a roller for a given job depends on the type of soil to be compacted and percentage
of compaction to be obtained. The types of rollers that are recommended for the soils normally
met are:
Type of soil Type of roller recommended
Cohesive soil Sheepsfoot roller, or Rubber tired roller
Cohesionless soils Rubber-tired roller or Vibratory roller.
Method of Compaction
The first approach to the problem of compaction is to select suitable equipment. If the
compaction is required for an earth dam, the number of passes of the roller required to compact
the given soil to the required density at the optimum moisture content has to be determined by
conducting a field trial test as follows:
The soil is well mixed with water which would give the optimum water content as
determined in the laboratory. It is then spread out in a layer. The thickness of the layer normally
varies from 15 to 22.5 cm. The number of passes required to obtain the specified density has to be
found by determining the density of the compacted material after every definite number of passes.
The density may be checked for different thickness in the layer. The suitable thickness of the
layer and the number of passes required to obtain the required density will have to be determined.

966 Chapter 21
In cohesive soils, densities of the order of 95 percent of standard Proctor can be obtained
with practically any of the rollers and tampers; however, vibrators are not effective in cohesive
soils. Where high densities are required in cohesive soils in the order of 95 percent of modified
Proctor, rubber tired rollers with tire loads in the order of 100 kN and tire pressure in the order of
600 kN/m2
are effective.
In cohesionless sands and gravels, vibrating type equipment is effective in producing
densities up to 100 percent of modified Proctor. Where densities are needed in excess of 100
percent of modified Proctor such as for base courses for heavy duty air fields and highways, rubber
tired rollers with tire loads of 130 kN and above and tire pressure of 1000 kN/m2
can be used to
produce densities up to 103 to 104 percent of modified Proctor.
Field Control of Compaction
Methods of Control of Density
The compaction of soil in the field must be such as to obtain the desired unit weight at the optimum
moisture content. The field engineer has therefore to make periodic checks to see whether the
compaction is giving desired results. The procedure of checking involves:
1. Measurement of the dry unit weight, and
2. Measurement of the moisture content.
There are many methods for determining the dry unit weight and/or moisture content of the
soil in-situ.The important methods are:
1. Sand cone method,
2. Rubber balloon method,
3. Nuclear method, and
4. Proctor needle method.
Sand Cone Method (ASTM Designation D-1556)
The sand for the sand cone method consists of a sand pouring jar shown in Fig. 21.12. The jar
contains uniformly graded clean and dry sand. A hole about 10 cm in diameter is made in the
soil to be tested up to the depth required. The weight of soil removed from the hole is
determined and its water content is also determined. Sand is run into the hole from the jar by
opening the valve above the cone until the hole and the cone below the valve is completely
filled. The valve is closed. The jar is calibrated to give the weight of the sand that just fills the
hole, that is, the difference in weight of the jar before and after filling the hole after allowing
for the weight of sand contained in the cone is the weight of sand poured into the hole.
Let
Ws = weight of dry sand poured into the hole
Gs = specific gravity of sand particles
W = weight of soil taken out of the hole
w = water content of the soil
Volume of sand in the hole = volume of soil taken out of the hole
W
that is, V = —?— (21 Aa)

Sand-cone
apparatus
3785 cm3
(1-gal)
II-*— 165 mm —H
U— 171 mm —*-l
165 mm
171 mm
ASTM dimensions
Mass of sand
to fill cone and
template groove
Base
template
(a)
Fig. 21.12 Sand-cone apparatus: (a) Schematic diagram, and (b) Photograph
(21.4b)
' " s
V
The dry unit weight of soil, yd -
W WGy
The bulk unit weight of soil, /= — = —
V Ws
rt
l+w
Rubber Balloon Method (ASTM Designation: D 2167)
The volume of an excavated hole in a given soil is determined using a liquid-filled calibrated
cylinder for filling a thin rubber membrane. This membrane is displaced to fill the hole. The in-
place unit weight is determined by dividing the wet mass of the soil removed by the volume of the
hole. The water (moisture) content and the in-place unit weight are used to calculate the in-place
dry unit weight. The volume is read directly on the graduated cylinder. Fig. 21.13 shows the
equipment.

968 Balloon
density apparatus
Chapter 21
Carrying
handle I
Rubber
membrane
Base
tempiate~~X
!/P%
[ — o
40
OQ
— 120
1480
1520
F— 1550
1
1600)
1
"S^^v
*« r"**"——"^
«
11
r>
II '( _
-1
-11
1
II
i
i1
II
II
III
II
II
Graduated
cylinder
(direct reading)
Hand
pump
Pump
control
valve
(a)
Figure 21.13 Rubber balloon density apparatus, (a) diagrammatic sketch, and
(b) a photograph
Nuclear Method
The modern instrument for rapid and precise field measurement of moisture content and unit
weight is the Nuclear density/Moisture meter. The measurements made by the meter are
non-destructive and require no physical or chemical processing of the material being tested. The
instrument may be used either in drilled holes or on the surface of the ground. The main advantage
of this equipment is that a single operator can obtain an immediate and accurate determination of
the in-situ dry density and moisture content.
Proctor Needle Method
The Proctor needle method is one of the methods developed for rapid determination of moisture
contents of soils in-situ. It consists of a needle attached to a spring loaded plunger, the stem of
which is calibrated to read the penetration resistance of the needle in lbs/in2
or kg/cm2
. The
needle is supplied with a series of bearing points so that a wide range of penetration resistances
can be measured. The bearing areas that are normally provided are 0.05, 0.1, 0.25, 0.50 and
1.0 sq. in. The apparatus is shown in Fig. 21.14. A Proctor penetrometer set is shown in Fig. 21.15
(ASTMD-1558).

T
Figure 21.14 Proctor needle Figure 21.15 Proctor penetrometer set (Courtesy:
Soiltest)
Laboratory Penetration Resistance Curve
A suitable needle point is selected for a soil to be compacted. If the soil is cohesive, a needle with
a larger bearing area is selected. For cohesionless soils, a needle with a smaller bearing area will
be sufficient. The soil sample is compacted in the mold.
OO
ao 1.
&i.
1.5

/
Penetrat
_
r

Moisture de
ion resistanc
~N
V
nsity curve
e curve

A
3 6 12 18 2
""^O
to
-P^
Os
0
O
O
O
C
Percentage
resistance,
kg/cm
2
Moisture content. %
Figure 21.16 Field method of determining water content by Proctor needle method

970 Chapter 21
The penetrometer with a known bearing area of the tip is forced with a gradual uniform
push at a rate of about 1.25 cm per sec to a depth of 7.5 cm into the soil. The penetration
resistance in kg/cm2
is read off the calibrated shaft of the penetrometer. The water content of the
soil and the corresponding dry density are also determined. The procedure is repeated for the
same soil compacted at different moisture contents. Curves giving the moisture-density and
penetration resistance-moisture content relationship are plotted as shown in Fig. 21.16.
To determine the moisture content in the field, a sample of the wet soil is compacted into the
mold under the same conditions as used in the laboratory for obtaining the penetration resistance
curve. The Proctor needle is forced into the soil and its resistance is determined. The moisture
content is read from the laboratory calibration curve.
This method is quite rapid, and is sufficiently accurate for fine-grained cohesive soils.
However, the presence of gravel or small stones in the soil makes the reading on the Proctor needle
less reliable. It is not very accurate in cohesionless sands.
Example 21.3
The following observations were recorded when a sand cone test was conducted for finding the unit
weight of a natural soil:
Total density of sand used in the test = 1.4 g/cm3
Mass of the soil excavated from hole = 950 g.
Mass of the sand filling the hole = 700 g.
Water content of the natural soil = 15 percent.
Specific gravity of the soil grains = 2,7
Calculate: (i) the wet unit weight, (ii) the dry unit weight, (iii) the void ratio, and (iv) the degree of
saturation.
Solution
Volume of the hole V = — =500 cm3
p
1.4
950
Wet density ofnatural soil, p. - - =1.9 g/cm3
oryt = 18.64 kN/m3
y
' 5 0 0 '
p 1.9
Dry density p, = —— = - =1.65 g/cm3
y y Hd
1 + w 1 + 0.15
f~! J "I
xl or 1.65+ 1.65^ = 2.7
w
A . ' w i
l+e l+e
Therefore e - —-'•— =0.64
1.65
„ vvG9 0.15x2.7x100 ^m
And S= --= - = 63%
e 0.64

Example 21.4
Old records of a soil compacted in the past gave compaction water content of 15% and saturation
85%. What might be the dry density of the soil?
Solution
The specific gravity of the soil grains is not known, but as it varies in a small range of 2.6 to 2.7, it
can suitably be assumed. An average value of 2.65 is considered here.
wG
s 0.15x2.65 nA^
Hence e = = = 0.47
S 0.85
f O A^
and dry density p, =—— p - —'• x 1 = 1.8 g/cm3
or dry unit weight = 17.66 kN/m3
d
l + e w
1 + 0.47 5
Example 21.5
The following data are available in connection with the construction of an embankment:
(a) soil from borrow pit: Natural density = 1.75 Mg/m3
, Natural water content = 12%
(b) soil after compaction: density = 2 Mg/m3
, water content = 18%.
For every 100 m3
of compacted soil of the embankment, estimate:
(i) the quantity of soil to be excavated from the borrow pit, and
(ii) the amount of water to be added
Note: 1 g/cm3
= 1000 kg/m3
= 103
x 103
g/m3
= 1 Mg/m3
where Mg stands for Megagram
106
g.
Solution
The soil is compacted in the embankment with density of 2 Mg/m3
and with 18% water content.
Hence, for 100 m3
of soil
Mass of compacted wet soil = 100 x 2.0 = 200 Mg = 200 x 103
kg.
200 200
Mass of compacted dry soil = = = 169.5 Mg = 169.5 x 103
kg
v 3
1 +w 1 +0.18 6 6
Mass of wet soil to be excavated = 169.5(1+ w) = 169.5(1+ 0.12) = 189.84 Mg
189 84
Volume of the wet soil to be excavated = '•— = 108.48 m3
1.75
Now, in the natural state, the moisture present in 169.5 x 103
kg of dry soil would be
169.5 x 103
x 0.12 = 20.34 x 103
kg
and the moisture which the soil will possess during compaction is
169.5 x 103
x 0. 18 = 30.51 x 103
kg
Hence mass of water to be added for every 100 m3
of compacted soil is
(30.51 - 20.34) 103
= 10.17 x 103
kg.

972 Chapter 21
Example 21.6
A sample of soil compacted according to the standard Proctor test has a density of 2.06 g/cm3
at
100% compaction and at an optimum water content of 14%. What is the dry unit weight? What is
the dry unit weight at zero air-voids? If the voids become filled with water what would be the
saturated unit weight? Assume Gs = 2.67.
Solution
Refer to Fig. Ex. 21.6. Assume V= total volume = 1cm3
. Since water content is 14% we may write,
—^ = 0.14 or M =0.14M
MS
and since, MW + MS = 2.06 g
0.14M +M =1.14M =2.06
s s s
2.06
or M = = 1.807 e
1.14
Mw = 0.14 x 1.807 = 0.253 g.
M, 1.807
By definition, pd =
v i
The volume of solids (Fig. Ex. 21.6) is
1.807
= 1.807 g/cmj
or yd = 1-807x9.81 = 17.73 kN/nr
V =
2.67
= 0.68 g/cnr
The volume of voids = 1- 0.68 = 0.32 cm3
The volume of water = 0.253 cm3
The volume of air = 0.320 - 0.253 = 0.067 cm3
If all the air is squeezed out of the samples the dry density at zero air voids would be, by
definition,
Air
Water
M w
Solids
Figure Ex. 21.6

1.807
= 1.94g/cm3
or y, = 1.94 x 9.81 = 19.03kN/m3
a
0.68+ 0.253
on the other hand, if the air voids also were filled with water,
The mass of water would be = 0.32 x 1 = 0.32g
The saturated density is
1.807 +0.32
Aat ~
1
= 2.13 g/cm-3
or ysat = 2.13x 9.81 = 20.90 kN/nr
21.6 COMPACTION FOR DEEPER LAYERS OF SOIL
Three types of dynamic compaction for deeper layers of soil are discussed here. They are:
1. Vibroflotation.
2. Dropping of a heavy weight.
3. Blasting.
Vibroflotation
The Vibroflotation technique is used for compacting granular soil only. The vibroflot is a
cylindrical tube containing water jets at top and bottom and equipped with a rotating eccentric
weight, which develops a horizontal vibratory motion as shown in Fig. 21.17. The vibroflot is sunk
into the soil using the lower jets and is then raised in successive small increments, during which the
surrounding material is compacted by the vibrationprocess. The enlarged hole around the vibroflot
is backfilled with suitable granular material. This method is very effective for increasing the density
of a sand deposit for depths up to 30 m. Probe spacings of compaction holes should be on a grid
pattern of about 2 m to produce relative densities greater than 70 percent over the entire area. If the
sand is coarse, the spacings may be somewhat larger.
In soft cohesive soil and organic soils the Vibroflotation technique has been used with gravel
as the backfill material. The resulting densified stone column effectively reinforces softer soils and
acts as a bearing pile for foundations.
Figure 21.17 Compaction by using vibroflot (Brown, 1977)

974 Chapter 21
Dropping of a Heavy Weight
The repeated dropping of a heavy weight on to the ground surface is one of the simplest of the
methods of compacting loose soil.
The method, known as deep dynamic compaction or deep dynamic consolidation may be
used to compact cohesionless or cohesive soils. The method uses a crane to lift a concrete or steel
block, weighing up to 500 kN and up to heightsof 40 to 50 m, from which height it is allowed to fall
freely on to the ground surface. The weight leaves a deep pit at the surface. The process is then
repeated either at the same location or sequentially over other parts of the area to be compacted.
When the required number of repetitions is completed over the entire area, the compaction at depth
is completed. The soils near the surface, however, are in a greatly disturbed condition. The top soil
may then be levelled and compacted, using normal compactiing equipment. The principal claims of
this method are:
1. Depth of recompaction can reach up to 10 to 12 m.
2. All soils can be compacted.
3. The method produces equal settlements more quickly than do static (surcharge type) loads.
The depth of recompaction, D, in meters is approximately given by Leonards, et al., (1980) as
(21.5)
where W = weight of falling mass in metric tons,
h = height of drop in meters.
Blasting
Blasting, through the use of buried, time-delayed explosive charges, has been used to densify loose,
granular soils. The sands and gravels must be essentially cohesionless with a maximum of 15
percent of their particles passing the No. 200 sieve size and 3 percent passing 0.005 mm size. The
moisture condition of the soil is also important for surface tension forces in the partially saturated
state limit the effectiveness of the technique.Thus the soil, as well as being granular, must be dry or
saturated, which requires sometimes prewetting the site via construction of a dike and reservoir
system.
The technique requires careful planning and is used at a remote site. Theoretically, an
individual charge densifies the surroundingadjacent soil and soil beneath the blast. It should not lift
the soil situated above the blast, however, since the upper soil should provide a surcharge load. The
charge should not create a crater in the soil. Charge delays should be timed to explode from the
bottom of the layer being densified upward in a uniform manner. The uppermost part of the stratum
is always loosened, but this can be surface-compacted by vibratory rollers. Experience indicates
that repeated blasts of small charges are more effective than a single large charge for achieving the
desired results.
21.7 PRELOADING
Preloading is a technique that can successfully be used to densify soft to very soft cohesive soils.
Large-scale construction sites composed of weak silts and clays or organic materials (particularly
marine deposits), sanitary land fills, and other compressible soils may often be stabilized
effectively and economically by preloading. Preloading compresses the soil. Compression takes
place when the water in the pores of the soil is removed which amounts to artificial consolidation of
soil in the field. In order to remove the water squeezed out of the pores and hasten the period of
consolidation, horizontal and vertical drains are required to be provided in the mass. The preload is

Filter bed
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(a) Vertical section (b) Section of single
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;/ x x v
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-_-x
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-_-x /
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(c) plan of sand drains
Figure 21.18 Consolidation of soil using sand drains
generally in the form of an imposed earth fill which must be left in place long enough to induce
consolidation. The process of consolidation can be checked by providing suitable settlement plates
and piezometers. The greater the surcharge load, shorter the time for consolidation. This is a case of
three-dimension consolidation.
Two types of vertical drains considered are
1. Cylindrical sand drains
2. Wick (prefabricated vertical) drains
Sand Drains
Vertical and horizontal and drains are normally used for consolidating very soft clay, silt and other
compressible materials. The arrangement of sand drains shown in Fig. 21.18 is explained below:
1. It consists of a series of vertical sand drains or piles. Normally medium to coarse sand is
used.
2. The diameter of the drains are generally not less than 30 cm and the drains are placed in a
square grid pattern at distances of 2 to 3 meters apart. Economy requires a careful study of
the effect of spacing the sand drains on the rate of consolidation.
3. Depth of the vertical drains should extend up to the thickness of the compressible stratum.

976 Chapter 21
4. A horizontal blanket of free draining sand should be placed on the top of the stratum and
the thickness of this may be up to a meter, and
5. Soil surcharge in the form of an embankmentis constructed on top of the sand blanket in
stages.
The height of surcharge should be so controlled as to keep the development of pore water
pressure in the compressible strata at a low level. Rapid loading may induce high pore water
pressures resulting in the failure of the stratum by rupture. The lateral displacement of the soil
may shear off the sand drains and block the drainage path.
The application of surcharge squeezes out water in radial directions to the nearest sand drain
and also in the vertical direction to the sand blanket. The dashed lines shown in Fig. 21.18(b) are
drawn midway between the drains. The planes passing through these lines may be considered as
impermeable membranes and all the water within a block has to flow to the drain at the center. The
problem of computing the rate of radial drainage can be simplified without appreciable error by
assuming that each block can be replaced by a cylinderof radius R such that
n R2
= L?
where L is the side length of the prismatic block.
The relation between the time t and degree of consolidation Uz% is determined by the
equation
Uz% = 100/(T)
wherein,
T =^4r (21.6)
H2
If the bottom of the compressible layer is impermeable, then H is the full thickness of the
layer.
For radial drainage, Rendulic (1935) has shown that the relation between the time t and the
degree of consolidation Ur% can be expressed as
Ur%=WOf(T) (21.7)
wherein,
is the time factor. The relation between the degree of consolidation U% and the time factor Tr
depends on the value of the ratio Rlr. The relation between Tr and U% for ratios of Rlr equal to 1,
10 and 100 in Fig. 21.19 are expressed by curves C;, C10 and Cwo respectively.
Installation of Vertical Sand Drains
The sand drains are installed as follows
1. A casing pipe of the required diameter with the bottom closed with a loose-fit-cone is
driven up to the required depth,
2. The cone is slightly separated from the casing by driving a mandrel into the casing, and
3. The sand of the required gradation is poured into the pipe for a short depth and at the same
time the pipe is pulled up in steps. As the pipe is pulled up, the sand is forced out of the pipe
by applying pressure on to the surface of the sand. The procedure is repeated till the holes
is completely filled with sand.

0.6
Time factor Tr
0.8 1.0 1.2
Figure 21.19 7" versus U
The sand drains may also be installed byjetting a hole in the soil or by driving an open casing
into the soil, washing the soil out of the casing, and filling the hole with sand afterwards.
Sand drains have been used extensively in many parts of the world for stabilizing soils for
port development works and for foundations of structures in reclaimed areas on the sea coasts. It is
possible that sand drains may not function satisfactorily if the soil surrounding the well gets
remolded. This condition is referred to as smear. Though theories have been developed by
considering different thickness of smear and different permeability, it is doubtful whether such
theories are of any practical use since it would be very difficult to evaluate the quality of the smear
in the field.
Wick (Prefabricated Vertical) Drains
Geocomposites used as drainage media have completely taken over certain geotechnical
application areas. Wick drains, usually consisting of plastic fluted or nubbed cores that are
surrounded by a geotextile filter, have considerable tensile strength. Wick drains do not require any
sand to transmit flow. Most synthetic drains are of a strip shape. The strip drains are generally 100
mm wide and 2 to 6 mm thick. Fig. 21.20 shows typical core shapes of strip drains (Hausmann,
1990).
Wick drains are installed by using a hollow lance. The wick drain is threaded into a hollow
lance, which is pushed (or driven) through the soil layer, which collapses around it. At the ground
surface the ends of the wick drains (typically at 1 to 2 m spacing) are interconnected by a granular
soil drainage layer or geocomposite sheet drain layer. There are a number of commercially
available wick drain manufacturers and installation contractors who provide information on the
current products, styles, properties, and estimated costs (Koerner, 1999).
With regards to determining wick drain spacings, the initial focal point is on the time for the
consolidation of the subsoil to occur. Generally the time for 90% consolidation (/90) is desired. In
order to estimate the time t, it is first necessary to estimate an equivalent sand drain diameter for the
wick drain used. The equations suggested by Koerner (1999) are
(21.9a)

978 Chapter 21
Filter
sleeve
Core
Fluted PVC
or paper
Various shapes
of cores with
nonwoven
geotextile
filter sleeves
V////////7l
• IHIIIUM+M
iinrmnnnrmnji
Figure 21.20 Typical core shapes of strip drains (Hausmann, 1990)
d.. = (21.9b)
where dsd = equivalent sand drain diameter
dv = equivalent void circle diameter
b,t = width and thickness of the wick drain
ns - porosity of sand drain
Void area of wick drain Void area of wick drain
- -
total cross sectional area of strip b x t
It may be noted here that equivalentsand drain diameters for various commercially available
wick drains vary from 30 to 50 mm (Korner, 1999).
The equation for estimating the time t for consolidation is (Koerner,1999)
8c, 1-U
(21.10)
where t = time for consolidation
ch = coefficient of consolidation of soil for horizontal flow

d = equivalent diameter of strip drain
_ circumference
n
D = sphere of influence of the strip drain;
a) for a triangular pattern, D = 1.05 x spacing Dt
b) for a square pattern, D = 1.13 x spacing D?
D( - distance between drains in triangular spacing and
Ds = distance for square pattern
U = average degree of consolidation
Advantages of Using Wick Drains (Koerner, 1999)
1. The analytic procedure is available and straightforward in its use.
2. Tensile strength is definitely afforded to the soft soil by the installation of the wick drains.
3. There is only nominal resistance to the flow of water if it enters the wick drain.
4. Construction equipment is generally small.
5. Installation is simple, straightforward and economic.
Example 21.7
What is the equivalent sand drain diameter of a wick drain measuring 96 mm wide and 2.9 mm thick that
is 92% void in its cross section? Use an estimated sand porosity of 0.3 for typical sand in a sand drain.
Solution
Total area of wick drain = b x t = 96 x 2.9 = 279 mm2
Void area of wick drain = nd x b x t - 0.92 x 279 = 257 mm2
The equivalent circle diameter (Eq. 21.9b) is
Ubtnd |4x257
d =, =, = 18.1 mm
v
V n V 3.14
The equivalent sand drain diameter (Eq. 21.19a) is
Example 21.8
Calculate the times required for 50, 70 and 90% consolidation of a saturated clayey silt soil using
wick drains at various triangular spacings. The wick drains measure 100 x 4 mm and the soil has a
ch =6.5 1Q-6
m2
/min.
Solution
In the simplified formula the equivalent diameter d of a strip drain is
circumference 100 + 100 + 4+4 ^ _
d = = 66.2 mm
n 3.14

980 Chapter 21
Using Eq. (21.10)
D2
D
t = in —-0.75 In
8c, d
substituting the known values
ln-°— 0.75 in
.
8(6.5 x!0~6
) 0.0062 1-U
The times required for the various degrees of consolidation are tabulated below for assumed
theoretical spacings of wick drains.
Wick drain
spacings
2.1
1.8
1.5
1.2
0.9
0.6
0.3
Time in days for various
degrees of consolidation(U)
50%
110
77
49
29
14
4.8
0.6
70%
192
133
86
50
24
8.4
1.1
90%
367
254
164
95
46
16
2.1
For the triangular pattern, the spacing Dt is
D=^~
' 1.05
21.8 SAND COMPACTION PILES AND STONE COLUMNS
Sand Compaction Piles
Sand compaction piles consists of driving a hollow steel pipe with the bottom closed with a
collapsible plate down to the required depth; filling it with sand, and withdrawing the pipe while air
pressure is directed against the sand inside it. The bottom plate opens during withdrawal and the
sand backfills the voids created earlier during the driving of the pipe. The in-situ soil is densified
while the pipe is being withdrawn, and the sand backfill prevents the soil surrounding the
compaction pipe from collapsing as the pipe is withdrawn.The maximum limits on the amount of
fines that can be present are 15 percent passing the No. 200 sieve (0.075 mm) and 3 percent passing
0.005 mm. The distance between the piles may have to be planned according to the site conditions.
Stone Columns
The method described for installing sand compaction piles or the vibroflot described earlier can be
used to construct stone columns. The size of the stones used for this purpose range from about 6 to
40 mm. Stone columns have particular application in soft inorganic, cohesive soils and are
generally inserted on a volume displacement basis.

The diameter of the pipe used either for the construction of sand drains or sand compaction
piles can be increased according to the requirements. Stones are placed in the pipe instead of sand,
and the technique of constructing stone columns remains the same as that for sand piles.
Stone columns are placed 1 to 3 m apart over the whole area. There is no theoretical
procedure for predicting the combined improvement obtained, so it is usual to assume the
foundation loads are carried only by the several stone columns with no contribution from the
intermediate ground (Bowles, 1996).
Bowles (1996) gives an approximate formula for the allowable bearing capacity of stone
columns as
1a=-jjL
(*C+
^ (21.11)
where Kp = tan2
(45° + 072),
0'= drained angle of friction of stone,
c - either drained cohesion (suggested for large areas) or the undrained shear strength c ,
Gr' = effective radial stress as measured by a pressuremeter (but may use 2c if
pressuremeter data are not available),
Fs = factor of safety, 1.5 to 2.0.
The total allowable load on a stone column of average cross-section area AC is
Stone columns should extend through soft clay to firm strata to control settlements. There is
no end bearing in Eq. (21.11) because the principal load carrying mechanism is local perimeter
shear.
Settlement is usually the principal concern with stone columns since bearing capacity is
usually quite adequate (Bowles, 1996). There is no method currently available to compute
settlement on a theoretical basis.
Stone columns are not applicable to thick deposits of peat or highly organic silts or clays
(Bowles, 1996). Stone columns can be used in loose sand deposits to increase the density.
21.9 SOIL STABILIZATION BY THE USE OF ADMIXTURES
The physical properties of soils can often economically be improved by the use of admixtures.
Some of the more widely used admixtures include lime, portland cement and asphalt. The process
of soil stabilization first involves mixing with the soil a suitable additive which changes its
property and then compacting the admixture suitably. This method is applicable only for soils in
shallow foundations or the base courses of roads, airfield pavements, etc.
Soil-lime Stabilization
Lime stabilization improves the strength, stiffness and durability of fine grained materials. In
addition, lime is sometimes used to improve the properties of the fine grained fraction of granular
soils. Lime has been used as a stabilizer for soils in the base courses of pavement systems, under
concrete foundations, on embankment slopes and canal linings.
Adding lime to soils produces a maximumdensity under a higher optimum moisture content
than in the untreated soil. Moreover, lime produces a decrease in plasticity index.
Lime stabilization has been extensively used to decrease swelling potential and swelling
pressures in clays. Ordinarily the strength of wet clay is improved when a proper amount of lime

982 Chapter 21
is added. The improvement in strength is partly due to the decrease in plastic properties of the clay
and partly to the pozzolanic reaction of lime with soil, which produces a cemented material that
increases in strength with time. Lime-treated soils, in general, have greater strength and a higher
modulus of elasticity than untreated soils.
Recommended percentages of lime for soil stabilizationvary from 2 to 10 percent. For coarse
soils such as clayey gravels, sandy soils with less than 50 per cent silt-clay fraction, the per cent of
lime varies from 2 to 5, whereas for soils with more than 50 percent silt-clay fraction, the percent of
lime lies between 5 and 10. Lime is also used with fly ash. The fly ash may vary from 10 to 20 per
cent, and the percent of lime may lie between 3 and 7.
Soil-Cement Stabilization
Soil-cement is the reaction product of an intimatemixtureof pulverized soil and measured amounts
of portland cement and water, compacted to high density. As the cement hydrates, the mixture
becomes a hard, durable structural material. Hardened soil-cement has the capacity to bridge over
local weak points in a subgrade. When properly made, it does not soften when exposed to wetting
and drying, or freezing and thawing cycles.
Portland cement and soil mixed at the proper moisture content has been used increasingly in
recent years to stabilize soils in special situations. Probably the main use has been to build
stabilized bases under concrete pavements for highways and airfields. Soil cement mixtures are
also used to provide wave protection on earth dams. There are three categories of soil-cement
(Mitchell and Freitag, 1959). They are:
1. Normal soil-cement usually contains 5 to 14 percent cement by weight and is used
generally for stabilizing low plasticity soils and sandy soils.
2. Plastic soil-cement has enough water to produce a wet consistency similar to mortar. This
material is suitable for use as water proof canal linings and for erosion protection on steep
slopes where road building equipment may not be used.
3. Cement-modified soil is a mix that generally contains less than 5 percent cement by
volume. This forms a less rigid system than either of the other types, but improves the
engineering properties of the soil and reduces the ability of the soil to expand by drawing
in water.
The cement requirement depends on the gradation of the soil. A well graded soil containing
gravel, coarse sand and fine sand with or without small amounts of silt or clay will require
5 percent or less cement by weight. Poorly graded sands with minimal amount of silt will require
about 9 percent by weight. The remaining sandy soils will generally require 7 percent. Non-plastic
or moderately plastic silty soils generally require about 10 percent, and plastic clay soils require 13
percent or more.
Bituminous Soil Stabilization
Bituminous materials such as asphalts, tars, and pitches are used in various consistencies to
improve the engineering properties of soils. Mixed with cohesive soils, bituminous materials
improve the bearing capacity and soil strength at low moisture content. The purpose of
incorporating bitumen into such soils is to water proof them as a means to maintain a low moisture
content. Bituminous materials added to sand act as a cementing agent and produces a stronger,
more coherent mass. The amount of bitumen added varies from 4 to 7 percent for cohesive
materials and 4 to 10 percent for sandy materials. The primary use of bituminous materials is in
road construction where it may be the primary ingredient for the surface course or be used in the
subsurface and base courses for stabilizing soils.

21.10 SOIL STABILIZATION BY INJECTION OF SUITABLE GROUTS
Grouting is a process whereby fluid like materials, either in suspension, or solution form, are
injected into the subsurface soil or rock.
The purpose of injecting a grout may be any one or more of the following:
1. To decrease permeability.
2. To increase shear strength.
3. To decrease compressibility.
Suspension-type grouts include soil, cement, lime, asphalt emulsion, etc., while the solution type
grouts include a wide variety of chemicals. Groutingproves especially effective in the following cases:
1. When the foundation has to be constructed below the ground water table. The deeper the
foundation, the longer the time needed for construction, and therefore, the more benefit
gained from grouting as compared with dewatering.
2. When there is difficult access to the foundation level. This is very often the case in city
work, in tunnel shafts, sewers, and subway construction.
3. When the geometric dimensions of the foundation are complicated and involves many
boundaries and contact zones.
4. When the adjacent structures require that the soil of the foundation strata should not be
excavated (extension of existing foundations into deeper layers).
Grouting has been extensively used primarily to control ground water flow under earth and
masonry dams, where rock grouting is used. Since the process fills soil voids with some type of
stabilizing material grouting is also used to increase soil strength and prevent excessive
settlement.
Many different materials have been injected into soils to produce changes in the engineering
properties of the soil. In one method a casing is driven and injection is made under pressure to the
soil at the bottom of the hole as the casing is withdrawn. In another method, a grouting hole is
drilled and at each level in which injection is desired, the drill is withdrawn and a collar is placed at
the top of the area to be grouted and grout is forced into the soil under pressure. Another method is
to perforate the casing in the area to be grouted and leave the casing permanently in the soil.
Penetration grouting may involve portland cement or fine grained soils such as bentonite or
other materials of a paniculate nature. These materials penetrate only a short distance through most
soils and are primarily useful in very coarse sands or gravels. Viscous fluids, such as a solution of
sodium silicate, may be used to penetrate fine grained soils. Some of these solutions form gels
that restrict permeability and improve compressibility and strength properties.
Displacement grouting usually consists of using a grout like portland cement and sand
mixture which when forced into the soil displaces and compacts the surrounding material about a
central core of grout. Injection of lime is sometimes used to produce lenses in the soil that will
block the flow of water and reduce compressibility and expansion properties of the soil. The
lenses are produced by hydraulic fracturing of the soil.
The injection and grouting methods are generally expensive compared with other
stabilization techniques and are primarily used under special situations as mentioned earlier. For a
detailed study on injections, readers may refer to Caron et al., (1975).
21.11 PROBLEMS
21.1 Differentiate: (i) Compaction and consolidation, and (ii) Standard Proctor and modified
Proctor tests.

984 Chapter 21
21.2 Draw an ideal 'compaction curve' and discuss the effect of moisture on the dry unit weight
of soil.
21.3 Explain: (i) the unit, in which the compaction is measured, (ii) 95 percent of Proctor
density, (iii) zero air-voids line, and (iv) effect of compaction on the shear strength of
soil.
21.4 What are the types of rollers used for compacting different types of soils in the field? How
do you decide the compactive effort requiredfor compacting the soil to a desired density in
the field?
21.5 What are the methods adopted for measuring the density of the compacted soil? Briefly
describe the one which will suit all types of soils.
21.6 A soil having a specific gravity of solids G = 2.75, is subjected to Proctor compaction
test in a mold of volume V = 945 cm3
. The observations recorded are as follows:
Observation number 1 2 3 4 5
Mass of wet sample, g 1389 1767 1824 1784 1701
Water content, percentage 7.5 12.1 17.5 21.0 25.1
What are the values of maximum dry unit weight and the optimum moisture content?
Draw 100% saturation line.
21.7 A field density test was conducted by sand cone method. The observation data are given
below:
(a) Mass ofjar with cone and sand (before use) = 4950 g, (b) mass ofjar with cone and sand
(after use) = 2280 g, (c) mass of soil from the hole = 2925 g, (d) dry density of
sand = 1.48 g/cm3
, (e) water content of the wet soil = 12%. Determine the dry unit weight
of compacted soil.
21.8 If a clayey sample is saturated at a water content of 30%, what is its density? Assume a
value for specific gravity of solids.
21.9 A soil in a borrow pit is at a dry density of 1.7 Mg/m3
with a water content of 12%. If a soil
mass of 2000 cubic meter volume is excavated from the pit and compacted in an
embankment with a porosity of 0.32, calculate the volume of the embankment which can
be constructed out of this material. Assume Gs = 2.70.
21.10 In a Proctor compaction test, for one observation, the mass of the wet sample is missing.
The oven dry mass of this sample was 1800 g. The volume of the mold used was 950 cm3
.
If the saturation of this sample was 80 percent, determine (i) the moisture content, and
(ii) the total unit weight of the sample. Assume Gs = 2.70.
21.11. A field-compacted sample of a sandy loam was found to have a wet density of
2.176 Mg/m3
at a water content of 10%. The maximum dry density of the soil obtained in
a standard Proctor test was 2.0 Mg/m3
. Assume Gs =2.65. Compute pd, S, n and the percent
of compaction of the field sample.
21.12 A proposed earth embankment is required to be compacted to 95% of standard Proctor dry
density. Tests on the material to be used for the embankment give pmax = 1.984 Mg/m3
at an
optimum water content of 12%. The borrow pit material in its natural condition has a void
ratio of 0.60. If G = 2.65, what is the minimum volume of the borrow required to make
1 cu.m of acceptable compacted fill?
21.13 The following data were obtained from a field density test on a compacted fill of sandy
clay. Laboratory moisture density tests on the fill material indicated a maximum dry

density of 1.92 Mg/m3
at an optimum water content of 11%. What was the percent
compaction of the fill? Was the fill water content above or below optimum.
Mass of the moist soil removed from the test hole = 1038 g
Mass of the soil after oven drying = 914 g
Volume of the test hole = 478.55 cm3
21.14 A field density test performed by sand-cone method gave the following data.
Mass of the soil removed + pan = 1590 g
Mass of the pan = 125 g
Volume of the test hole = 750 cm3
Water contentinformation
Mass of the wet soil + pan = 404.9 g
Mass of the dry soil + pan = 365.9 g
Mass of the pan = 122.0 g
Compute: pd, yd, and the water content of the soil. Assume G5 = 2.67

Chapter 21

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